1. Field of Invention
This invention relates to electronic measurement equipment, more specifically to portable equipment for making in-situ harmonic voltage measurements and harmonic current measurements on alternating current (AC) power conductors. The equipment performs an in-situ calibration of its measurement channels, including their associated current transducers, immediately before making an in-situ measurement.
2. Background of the Invention
Voltages and currents on electric power distribution grids generally operate at 50 Hertz or 60 Hertz. This frequency is called the fundamental frequency. In an ideal grid, the voltages and currents will be perfectly sinusoidal with respect to time. However, in real grids, the voltages and currents are distorted, and contain both a fundamental frequency component and numerous smaller harmonic components. The harmonic components are typically caused by nonlinear loads at commercial or industrial sites.
It is useful to determine the direction of harmonic propagation on electric power distribution grids. Knowing the direction of harmonic propagation may allow utility companies to detect the contribution of a particular user to harmonic distortion.
Typically, harmonic direction finding algorithms rely, at least in part, upon an accurate measurement of the phase angle between a voltage harmonic waveform and current harmonic waveform.
The inductive nature of typical power distribution systems means that this phase angle is often close to 90 degrees, so even a small error in measuring this parameter can lead to an apparent reversal of the direction of harmonic power flow, because the direction of flow reverses when the angle reaches 90 degrees. For example, if the phase angle is 89.5 degrees, the direction of harmonic power flow is in one direction; if the phase angle is 90.5 degrees, the direction of harmonic power flow is in the opposite direction. For this reason, an error of just one degree can lead to exactly the wrong conclusion about the direction of harmonic power flow.
Furthermore, a phase angle error of just 0.5 degrees at the fundamental frequency, caused by phase delay, translates mathematically to a much worse phase angle error of 24.5 degrees at the 49th harmonic, for example.
For these reasons, it is important to measure harmonic phase angles accurately.
A significant source of phase angle measurement error, as well as ratio error, comes from the split-core, clamp-on current transducers commonly used to measure in-situ current waveforms.
Clamp-on transducers are often required for practical in-situ measurements using portable instruments, because this type of current transformer does not require disconnecting or shutting down the load being measured. However, as known to those familiar with the art, this type of sensor introduces phase angle and ratio errors.
Some of these current transducer errors are constant for a given transducer, and can be calibrated at a factory or a laboratory. For example, some of the sources of ratio errors and phase errors associated with a particular current transducer can be calibrated at a factory or laboratory. An example of such a laboratory calibration was disclosed by McEachern (the first-named inventor in the present application) in U.S. Pat. No. 5,014,229.
However, other current transducer errors are a function of quantities that vary with time and location, such as temperature, humidity, the amount of corrosion on the mating surfaces of the magnetic core, the amount of mechanical wear on the hinge of the jaws, the smoothness of the mating surfaces of the magnetic core, and the spring pressure holding the jaws closed.
These transducer errors cannot be calibrated at a factory or laboratory, because they vary with time and location.
Additional errors are introduced in the harmonic measurements by the chain of electronics between the current transducer and the measurement results: amplifiers, filters, analog-to-digital converters, and the like.
Again, some of the errors in this chain of electronics can be calibrated at a factory or laboratory. The parts that are stable over time of the overall gain, offset, amplitude response, and phase response, for example, might be calibrated at a factory or laboratory.
However, some errors in this chain of electronics vary with temperature, humidity, time, ambient magnetic field, and other local parameters which may be known or unknown. These errors cannot be calibrated at a factory or laboratory.
As is familiar to one familiar with the state of the art, similar errors arise in the chain of electronics needed for voltage harmonic measurements.
For these reasons, to make precise in-situ measurements of harmonic voltages and harmonic currents, and the phase angles between harmonic voltages and currents, it is necessary to calibrate the entire measurement voltage and current channels, including the current transducers, immediately before making measurements. It is necessary to perform in-situ calibration of the current transducers at the actual measurement site, as the temperature and humidity may be different from those at a prior site and as the amount of corrosion on the mating surfaces of the magnetic core, and the amount of mechanical wear on the hinge of the jaws may have changed since calibration at the previous site, or at the initial calibration lab.
Harmonic voltage and current measuring instruments known in the art often have several channels of measurements. For example, a typical instrument may have three current channels of measurement, one for each phase on a three-phase AC power grid, plus four voltage channels of measurements, one for each phase-to-neutral voltage on a three-phase AC power grid and one for the neutral-to-ground voltage on the grid.
To maximize accuracy and minimize errors of all types, measurement channels known in the art are constructed from highly stable electronic components. As is well known in the art, components that affect the amplitude and phase response of measurement channels, such as resistors and capacitors, have values that vary with age, temperature, humidity, and other influencing factors. Selecting and purchasing components that are stable with respect to age, temperature, and humidity is difficult and expensive.
Different approaches have been taken in the art to perform calibration of energy meters or power monitoring systems, which are affected by similar sources of error as the harmonics measurement instrument. For example, Burns et al. in U.S. Pat. No. 6,377,037 disclose a meter with a power factor compensation technique that inserts a delay into the sampled current or voltage stream; however, Burns et al. do not disclose in-situ calibration for field measurements, and therefore cannot compensate for variations in sensors and amplifiers that would have developed since the instrument's most recent laboratory calibration.
Also, Gandhi in U.S. Pat. No. 6,911,813 discloses an electronic meter that delays the digital voltage and/or current signal to compensate for phase shift error; but Gandhi does not disclose in-situ calibration for field measurements, and therefore cannot compensate for variations in sensors and amplifiers that would have developed since the meter's most recent laboratory calibration.
Voisine et al. in U.S. Pat. No. 5,231,347 disclose a power meter that adjusts the phase angle between the voltage and current signals by adding a phase lead to the primary coil of the voltage transformer. Although this disclosure recognizes the problem of measuring the phase angle between voltage and current accurately, Voisine et al. do not disclose in-situ calibration for field measurements, and therefore cannot compensate for variations in sensors and amplifiers that would have developed since the power meter's most recent laboratory calibration.
Whitehead et al. in U.S. Pat. No. 6,639,413 takes the approach of including a test unit for applying a test signal to the system and also a phase reference module. However, Whitehead et al. do not disclose in-situ calibration using non-sinusoidal test input signals, i.e., those that contain harmonic content.
Longini in U.S. Pat. No. 5,325,048 utilizes a calibration stand comprising a current and voltage generator for generating standardized current and voltage signals. However, Longini does not disclose a calibration unit that is built into the meter itself and that performs calibration at the site where the power meter will be installed. Furthermore, Longini does not disclose a method to perform phase angle calibration.
In summary, none of these methods disclose in-situ calibration, at harmonic frequencies, of both the amplitude response of the current and voltage measurement circuits, including their associated current transducers and voltage probes, and the phase angle response between them, at the measurement sites immediately before making measurements.